This invention relates to a motor vehicle fuel tank and method of making a motor vehicle fuel tank.
Plastic fuel tanks for motor vehicles are typically manufactured by processes identified as xe2x80x9cblow moldingxe2x80x9d and xe2x80x9ctwin sheet thermoformingxe2x80x9d. In the blow molding process, a mass of liquid plastic at elevated temperature is expanded in a mold by injecting gas under pressure into the plastic mass. In the twin sheet thermoforming process, a pair of flat plastic sheets are vacuum formed in mold cavities at an elevated forming temperature to the shape of upper and lower shells each having an attachment flange around an open end thereof. When the mold cavities are closed, the attachment flanges are squeezed together to fusion bond the shells and thereby form a hollow plastic tank body. Plastic fuel tanks manufactured by both processes are subject to pressure and vacuum cycles attributable to differences between atmospheric pressure around the tank body and the pressure of a gaseous mixture of air and fuel vapor in the tank body. In a pressure cycle, gas pressure in the tank body exceeds atmospheric pressure and the top of the tank body expands away from the bottom of the tank body. In a vacuum cycle, atmospheric pressure exceeds the gas pressure in the tank body and the top of the tank body collapses toward the bottom of the tank body. To reinforce a blow molded plastic tank fuel tank against such pressure and vacuum cycles, it is known to install rigid structural supports through an access port in the top of the tank body. Such rigid supports may not be adequate, however, because of their location only in the immediate vicinity of the access port. To reinforce a twin sheet thermoformed plastic fuel tank, it is known to interpose a rigid structural support between the vacuum formed shells before the mold cavities are closed so that after the attachment flanges are fusion bonded, shrinkage during curing of the tank body fusion bonds the ends of the structural support to the top and bottom of the tank body. Such rigid structural supports, however, may compromise the integrity of the tank body by piercing the top and/or bottom thereof as the tank body shrinks.
This invention is a new and improved twin sheet thermoformed plastic fuel tank including a hollow plastic tank body and a variable length plastic strut in the hollow tank body. A pair of flat plastic sheets are vacuum formed at an elevated forming temperature in upper and lower mold cavities of a molding apparatus to the shape of upper and lower plastic shells. The plastic strut includes an upper foot, a lower foot, and a coupling means operable to support the upper foot on the lower foot for back and forth linear translation in the direction of a longitudinal centerline of the strut against a controlled resistance. With the mold cavities separated, the plastic strut is interposed between the upper and lower shells. Then, the mold cavities are closed together to form the hollow tank body by fusion bonding the upper and lower shells at respective attachment flanges on each. When the tank body thereafter cures and shrinks, the plastic strut is squeezed between the upper and lower shells against the controlled resistance of the coupling means to fusion bond the upper and lower feet of the strut to the shells. The strut then expands and collapses with the hollow tank body during pressure and vacuum cycles therein The coupling means also limits linear collapse and expansion of the strut to prevent collapse and expansion of the tank body which could negatively affect the structural integrity thereof. In a first embodiment of the plastic fuel tank according to this invention, a resilient flexible lobe of the plastic strut constitutes the coupling means. In a second embodiment of the plastic fuel tank according to this invention, a pilot on an upper shank portion of the strut, a guide on a lower shank portion of the strut for the pilot, and end stops on the upper and lower shank portions constitute the coupling means.